Sustainability Concept House
The Concrete Centre, together with registered state landlord A2Dominion Group, has developed a concrete and masonry home that achieves level 5 of the Code for Sustainable Homes. The proposal provides a new approach for high performance, low cost social housing Since the introduction of the Code for Sustainable Homes in 2006, a number of demonstration projects and developments have shown how to meet tough performance standards, up to level 4 and higher. The challenge now is to translate those solutions for mainstream housing and the public sector. For social housing this challenge is particularly demanding since increased energy efficiency must not have an impact on the core design requirements of affordability, low maintenance, simplicity, use of systems with a proven track record or the provision of robust and durable construction.
These concept designs take full account of this, and in addition meet current public social housing design standards such as Lifetime Homes, Building for Life and the augmented space standards. Furthermore, the designs not only take account of current legislation but also future foreseeable changes to the Standard Assessment Procedure (SAP) which is likely to take more account of thermal mass. A major consideration in meeting these requirements was the determination to use readily available and proven methods of concrete and masonry construction and design solutions that meant the home could be built in any location. The concept designs are for a five person, three-bedroom house of two storeys with a floor area of 85sq m. The plot was assumed to have a north/south orientation and low flood risk. For the ground floor, two types of beam and block system were considered, one using polystyrene blocks and the other concrete blocks. Both had a 90mm screed finish containing under floor heating pipes. To achieve a design U-value of 0.12W/m2K, the concrete block option requires around 200mm of insulation below the screed, whilst the polystyrene option only needs an additional 50mm of insulation, resulting in a much lower overall depth. However, this was not considered to be a particular design issue for the project. Other differences centred on cost and environmental rating, for which the polystyrene option scores an A+ in the BRE Green Guide. A concrete beam and block option scores a B, although the potential exists to upgrade this to an A rating by specifying screed with a high recycled content. With little to choose between the two options, concrete blocks were ultimately selected.
The design U-value of 0.15W/m2K for the external walls was achieved using a 200mm cavity fully filled with polystyrene beads and medium density aggregate block.
Had wall thickness and lower thermal conductivity been more of a driver, the greater insulating properties of aircrete blocks would have reduced the depth by some 10mm, or 15mm if using a thin-joint system. Concrete blocks, timber studs and steel studwork were all considered for the internal walls, but a combination of factors lead to the specification of block walls. These included a tougher finish, greater mass and good fire and floor resilience. It also enabled the preferred option of a wet plaster finish to be applied both externally and internally. The pitched roof provides optimum performance in terms of weathering, whole life costs, and the use of solar panels to achieve code level 5 targets. 450mm of mineral wool was determined to be the most economical way of achieving the required U-value of 0.12W/m2K. A traditional roof covering was a fundamental design requirement in order to meet the aspirations of the RSL for architectural designs that are acceptable to planning authorities on a national basis. Timber and UPVC windows were evaluated for thermal performance, maintenance requirements and durability. Despite lower levels of embodied CO2, timber was ruled out due to higher maintenance requirements — an issue of importance to the RSL. The decision to use UPVC windows was also influenced by their recent Green Guide environmental rating improvement from C to A to reflect the material’s good whole life performance and ability to be recycled at the end of life.
While thermal mass does not fall within the scope of the Code, making use of it was an important design requirement in order to reduce the risk of summertime overheating and so provide a degree of adaptation to our warming climate.
Depending on orientation, it also has the potential to improve energy efficiency during the heating season. Accordingly, the design optimizes the inherent thermal mass of the materials used helped by a wet plaster finish on the walls and a tiled floor where practicable.
The main RSL design requirements influencing the choice of heat source and renewable technologies centered on the following issues: ease of maintenance, cost, and use of a known technology, code score, and compatibility with other aspects of the design. These design requirements lead to the selection of a photovoltaic (PV) system and a class 5 gas condensing boiler, which reduce the case also including a solar hot water system. Instead, the entire renewable energy requirement is met by a 27sq m PV array. Having a single renewable technology reduces the overall complexity and maintenance requirements of the house. The concept designs from The Concrete Centre clearly demonstrate that existing trade skills can easily achieve code level 5.
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